This invention relates to a process for producing a magnetic recording medium, a magnetic paint and a magnetic disk, and particularly to a process for producing a magnetic recording medium having a good wear resistance suitable to applications such as magnetic disks, etc., a magnetic paint, and a magnetic disk.
Generally, in magnetic recording media such as magnetic disks, etc., a magnetic film is formed on various substrates, and the surface of the magnetic film is susceptible to frictions with a magnetic head at a high speed during the recording and reproduction. Deterioration of the properties of magnetic recording media by the frictions is a serious problem.
In order to protect the surface of a magnetic film from damages due to the frictions with a magnetic head, a lubricant has been usually applied to the surface of the magnetic film. It is known that fluorinated oils such as perfluoropolyether, etc. are preferable lubricants for the magnetic recording media, as disclosed in U.S. Pat. No. 3,778,306.
In the application of the lubricant to magnetic recording media such as magnetic disks, etc., the durability of the magnetic recording media is increased with increasing amount of the lubricant, but the sticking force of the magnetic head to the surface of the magnetic recording head is increased during the discontinuation to drive the magnetic recording apparatus when too much lubricant is applied to the surface of the magnetic film, and the magnetic head or the surface of the magnetic recording media is damaged at the start to drive the magnetic recording apparatus or the magnetic recording apparatus cannot be driven. The sticking force between the magnetic head and the magnetic recording medium depends upon the amount of the lubricant existing on the surface of the magnetic recording medium, as mentioned above, and it has been proposed to appropriately control the amount of a lubricant on the surface of a magnetic film by making the magnetic film porous and impregnating the pores of the porous magnetic film with the lubricant. For example, Japanese Patent Application Kokai (Laid-open) No. 53-104202 discloses a method comprising adding nonmagnetic particles to a magnetic film, thereby making the magnetic film porous and impregnating the resulting pores with a lubricant, and Japanese Patent Application Kokai (Laid-open) No. 63-181121 discloses a method comprising adding poly(alkyleneoxide) of good thermal decomposability to a magnetic paint, thermally decomposing the poly(alkyleneoxide) when the coated film is cured, thereby dissipating the decomposition products and making the magnetic film porous, and impregnating the pores of the porous magnetic film with a lubricant.
In the case of magnetic recording media of high recording density, where thinner magnetic films are required, Japanese Patent Application Kokai (Laid-open) No. 63-19325 discloses a method and a structure of impregnating an underlayer with a large amount of a lubricant and supplying the lubricant from the underlayer to the surface of a magnetic film through micropores in the magnetic film (the method will be hereinafter referred to as underlayer impregnation). It is further disclosed that the underlayer impregnation structure can be formed by adding poly(alkyleneoxide) of good thermal decomposability to an underlayer paint, applying the underlayer paint to a substrate as an underlayer, curing the underlayer at such a temperature as not to thermally decompose the poly(alkyleneoxide), then applying a magnetic film to the surface of the underlayer, and heat curing the magnetic film, thereby thermally decomposing the poly(alkyleneoxide) in the underlayer and dissipating the decomposition products at the same time and making the underlayer and the magnetic film porous.
In the foregoing prior art, the method of adding magnetic particles to the magnetic film has such problems as an increase in noise output due to the additive, etc. The method of making the magnetic film porous by thermal decomposition of the organic additive has such problems as formation of larger pores in the magnetic film when an additive having an appropriate compatibility with a binder in the magnetic paint is not properly selected, with the results of generation of noises or errors. Even the method of making the magnetic film porous by thermal decomposition of poly(alkylenoxide) still has such problems that the compatibility of poly(alkyleneoxide) with a binder in the magnetic paint is not satisfactory yet, though the poly(alkylenoxide) is selected in view of the thermal decomposition characteristics and the compatibility with the binder in the magnetic paint, and the procedures for dispersing magnetic particles, magnetic orientation, etc. have an influence on the phase separation, resulting in poor product yield.
Formation of underlayer impregnation structure of the prior art has another problem. In order to make the underlayer porous, it is necessary to add a thermally decomposable additive to the underlayer, as mentioned above. The underlayer is adjusted so that the binder or pigment in the underlayer may not enter the magnetic film by primary curing of the underlayer before the application of the magnetic film. However, since the thermally decomposable additive exists in a free state in the underlayer, it permeates into the magnetid film. When the additive permeates too much, the same effect is obtained at that when a large amount of the thermally decomposable additive is added to the magnetic paint, and the magnetic film becomes more porous than required.